Published 4:00 am, Wednesday, November 29, 2006

Quake team catches show on sea's floor / Scientists place gear in exact spot where magma erupted

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Ocean scientists have caught the Earth's crust in the very act of ripping itself apart when swarms of tiny undersea earthquakes ended in a volcanic eruption that buried their instruments in lava.

For marine geophysicist Maya Tolstoy, the event was the greatest thrill in nine years of research at sea, and for her and her colleagues it was an unparalleled opportunity to learn firsthand how the restless Earth behaves when stresses grow along a vulnerable stretch of crust at the bottom of the ocean.

All around the world, great slabs of that crust are constantly grinding against each other, just as tectonic plates do along California's San Andreas Fault. Nowhere is the phenomenon more active than along a midocean ridge called the East Pacific Rise, about 400 miles off the coast of Central and South America.

For decades, scientists have observed evidence of past episodes of "sea-floor spreading" in that area, as a rift in the Earth's crust 8,000 feet beneath the surface has sent up lava to create fresh crust and provide new evolutionary habitats for marine creatures capable of thriving in water as hot as 800 degrees Fahrenheit.

Tolstoy is a research scientist at Columbia University's Lamont-Doherty Earth Observatory, and three years ago she and her seagoing colleagues deployed a dozen ocean-bottom seismometers around 6 square miles of the ridge to study seismic activity there.

The new instruments recorded swarms of "microquakes" -- none of more than magnitude 1.5 or so. The swarms grew more and more intense until they were coming at a rate of nearly 2,500 a day by last year, and Tolstoy predicted that soon a burst of magma would erupt through the rift in the crust at that very spot.

It did.

In April, a group of her colleagues aboard the research vessel Knorr from the Woods Hole Oceanographic Institution in Massachusetts sailed over Tolstoy's site and stopped to recover the seismometers. To their surprise, only four of the 12 devices popped up to the surface in response to a signal from the ship, while three responded to the signal but failed to come up. The other five remained silent.

The four seismometers that did survive, however, had recorded the growing intensity of the earthquake swarms, as well as a continued series of humming vibrations that geophysicists call "harmonic tremors" caused by the magma rising into the undersea volcanic vent.

In May, after the scientists aboard the Knorr finished their job, another oceanographic ship, the New Horizon from the Scripps Institution of Oceanography at UC San Diego, towed its underwater camera over the site. The images revealed that the eight missing seismometers were all stuck fast, overwhelmed by fresh lava from the eruption.

And all that month, the ship's instruments noted, the water just above the half-buried instruments remained hot, darkened and polluted with methane in the aftermath of the eruption. Calculations showed that the eruption where the sea floor had split apart had thrust up a solid mass of magma for six hours straight on Jan. 22, Tolstoy and her colleagues reported.

The fresh lava apparently flowed along the ridge for more than 10 miles, she said, and the cloud of darkened water had spread south for 4 miles. Basaltic rocks dredged later from the eruption site were glassy and iridescent, evidence that the hot magma had cooled rapidly by contact with the seawater.

"This was the first time we've ever been able to catch a sea-floor spreading event just as it happened," Tolstoy said in an interview Tuesday. "It was the most phenomenally exciting event in my career -- evidence I'd always hoped to gather, and if it meant losing a few seismometers on the bottom of the ocean, this was certainly the best way to lose them."

With the instruments placed so fortunately right at the site of the eruption, Tolstoy said, the information she and her colleagues gleaned has provided new details about the phenomenon of sea-floor spreading.

Tolstoy has been the leader of a research group that included scientists from the University of Hawaii, the Pacific Marine Environmental Laboratory in Seattle, the Woods Hole Oceanographic Institution, Brown University, the University of Washington and the University of Florida. The group reported on its first-ever observations in the current issue of the journal Science.

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